Optical bandpass filter using long period gratings

ABSTRACT

The specification describes an optical filter wherein long period gratings (LPGs) are used in a new configuration to provide in-line bandpass filtering of optical signals. In the basic embodiment, two dissimilar LPGs are installed in the optical fiber transmission line. The first LPG converts light, over a broad wavelength range, being transmitted in the fundamental, or LP 01 , mode of the optical fiber transmission line into light in a higher order mode (HOM) of the optical fiber. The mode-converted signal, with mode LP m,n , is then coupled to a second waveguide, the second waveguide having transmission characteristics different from those of the first. The second waveguide supports propagation of light in a LP m,n  mode. The mode-converted signal is then transmitted through a second LPG where the signal over a selected narrow band of wavelengths that is accepted by the second LPG is converted back to a LP 01  mode. The selected narrow band in the LP 01  mode propagates efficiently over the remainder of the optical fiber transmission path.

RELATED APPLICATIONS

[0001] This application claims the benefit of provisional application60/301,164 filed Jun. 27, 2001, which is incorporated herein byreference.

FIELD OF THE INVENTION

[0002] This invention relates to optical filters, and more particularlyto optical bandpass filters with low loss.

BACKGROUND OF THE INVENTION

[0003] Optical bandpass filers transmit light over a pre-determined bandof wavelengths while rejecting, by absorption, radiation or scattering,all other wavelengths. Such filters are useful in laser cavities oroptical communications systems. For example, they may be used toconstrain the wavelength of operation of a laser, when deployed insideor outside the laser cavity. In optical communications systems, they canbe used at the input of an optical receiver to separate unwanted lightsuch as spontaneous emission noise outside the wavelength band of thesignal. See D. M. Shamoon, J. M. H. Elmirghani, R. A. Cryan,“Characterisation of optically preamplified receivers with fibre Bragggrating optical fibers”, IEEE Colloquium on Optical Fiber Gratings,March 1996. Optical regenerators based on self-phase modulation requireextracting a predetermined wavelength band from a broad spectrum oflight.

[0004] Several devices have been proposed and demonstrated to offer thefunctionality of bandpass filtering. Fiber Bragg gratings may be used inthe reflection mode with a circulator, or in the transmission mode, toselect a narrow wavelength band. See Xu, M. G.; Alavie, A. T. ;Maaskant, R.; Ohn, M. M.; “Tunable fiber bandpass filter based on alinearly chirped fiber Bragg grating for wavelength demultiplexing”,Electron Lett., 32, pp.1918-1919 (1996). Operation in the reflectionmode requires addition of a circulator in the transmission line—thisincreases cost, loss and complexity for the device. A further drawbackwith Bragg gratings, used either in transmission or reflection, is thatsuch filters can be highly dispersive, which gives rise to pulse shapedistortions. See Lenz, G.; Eggleton, B. J.; Giles, C. R.; Madsen, C. K.;Slusher, R. E.; “Dispersive properties of optical filters for WDMsystems”, IEEE Journal of Quantum Electronics, 34, pp. 1390-1402,(1998). Alternatively, thin film dielectric filters may also be used asbandpass filters (see: U.S. Pat. No. 5,615,289), but in addition totheir dispersive nature (similar to Bragg gratings) they suffer from theadditional drawbacks of being free-space devices. Thus, light needs tobe coupled into fiber pigtails for use in such fiber-optic systems. Thisincreases loss, cost and complexity.

[0005] An alternative technique for making bandpass filters uses twoidentical long-period fiber gratings (LPGs) that are spliced in seriesin the fiber-optic transmission line, with a core block between them.(See: U.S. Pat. No. 6,151,427.) The first long period grating converts anarrow wavelength-band of core-mode light into a cladding mode, and thesecond identical grating couples the cladding-mode light back into thecore mode. The core block between the two LPGs attenuates or scattersany light that was not converted into the cladding mode. There aredrawbacks associated with this device—(1) the core block simultaneouslyattenuates light in all the modes, thus the device is inherently lossy.Consequently only higher-order cladding modes may be utilized, as lowerorder cladding modes will exhibit even higher loss; (2) the core blockforms a discrete discontinuity in the fiber, which leads to undesiredmode-coupling and inter-modal interference; (3) tuning such a filterrequires simultaneously tuning both LPGs by identical amounts, as thefilter operates properly only when both LPGs have identical spectra. Inbandpass filters using acousto-optically generated or microbend-inducedLPGs, there is an additional drawback: the device is inherentlypolarization sensitive.

[0006] Thus, there exists a need for a bandpass filter that is anin-line fiber device, has low loss, is polarization insensitive,tunable, and simple to implement.

STATEMENT OF THE INVENTION

[0007] According to the invention, LPGs are used in a new configurationto provide in-line band pass filtering of optical signals in an opticalsystem. In a preferred case the optical system is an optical fibersystem. In the basic filter embodiment of the invention, two dissimilarLPGs are installed in serially cascaded optical waveguides of thetransmission line. In the first waveguide of the filter, the signallight is initially transmitted in the fundamental, or LP₀₁ mode. Thefirst LPG converts the signal light, over a broad wavelength range, intoa higher order mode (HOM) of the first optical waveguide. The conversionis achieved using a broadband LPG that provide strong mode conversionover a broad spectrum. See. S. Ramachandran, M. F. Yan, L. C. Cowsar, A.C. Carra, P. Wisk, R. G. Huff and D. Peckham, “Large bandwidth, highlyefficient mode coupling using long-period gratings in dispersiontailored fibers, ” Optics Letters,vol. 27, pp. 698-700 (2002); U.S. Pat.No. 6,084,996, both incorporated by reference herein. In the followingdescription the broadband LPG is designated BB-LPG. The BB-LPG operatesby exciting either a core guided HOM or a cladding guided HOM of thefirst optical waveguide. The mode-converted signal, with mode LP_(m,n),is then coupled to the second optical waveguide, the second waveguidehaving transmission characteristics different from those of the firstoptical waveguide. The second waveguide strongly couples to the LP_(m,n)mode, because the second LPG, which may be a conventional LPG, providesa strong narrow band coupling between LP_(m,n) and LP₀₁ modes of thesecond optical waveguide. The second LPG is referred to here as a narrowband LPG (NB-LPG).

[0008] Light entering the dual LPG filter first encounters the BB-LPGwhere more than 99% of the signal is converted to a LP_(m,n) mode over abroad wavelength range. When the converted signal encounters the NB-LPG,the signal over a selected narrow band of wavelengths accepted by theNB-LPG is converted back to the LP₀₁ mode of the second waveguide. Theselected narrow band in the LP₀₁ mode propagates efficiently over theremainder of the optical waveguide transmission path; i.e., theremainder of the second waveguide.

[0009] The mode distribution profiles of the LP_(m,n) mode excited inthe BB-LPG and the LP_(m,n) converted in the NB-LPG do not have tomatch, as long as the order of the modes (m and n) are the same. TheBB-LPG and the NB-LPG may comprise separate waveguide sections, or maybe formed in same length of waveguide. The waveguides are preferablyoptical fibers. If the LPGs comprise separate optical fibers splicedtogether the signal adiabatically couples at the splice since the modeshapes are the same.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a schematic diagram of the bandpass filter of theinvention;

[0011]FIG. 2 is a plot of grating period vs. wavelength showing therelationship for two LPGs in different optical fibers;

[0012]FIG. 3 is a plot of intensity vs. wavelength showing the effectivemode conversion properties for the BB-LPG and the NG-LPG;

[0013]FIG. 4 is a plot similar to that of FIG. 3 for the filter of FIG.1, i.e. with the combined LPGs;

[0014]FIG. 5 is a schematic diagram illustrating the use of the opticalfilter of the invention in a laser;

[0015]FIG. 6 is a schematic diagram illustrating the use of the opticalfilter of the invention in a Raman laser;

[0016]FIG. 7 is a schematic illustration of the use of the opticalfilter of the invention in an optical signal regenerator device; and

[0017]FIG. 8 is a schematic representation of a system using the opticalfilter of the invention in an optical preamplifier for an opticalreceiver.

DETAILED DESCRIPTION

[0018] Referring to FIG. 1, the arrangement shown represents the genericform of the bandpass filter of the invention. The filter comprises afirst fiber section 11 with a BB-LPG 12, a second fiber section 13, witha NB-LPG 14, and a splice 15 connecting the two fiber sections together.The splice 15 adiabatically converts the HOM of fiber 11 to the HOM offiber 13. The BB-LPG 12 has a grating period selected for broad-bandmode conversion, and the NB-LPG 14 has a different grating period chosenfor narrow band, i.e., bandpass, mode conversion. The choice of gratingperiods is illustrated in FIG. 2.

[0019] With reference to FIG. 2, phase-matching relationships are shownfor LPGs coupling the LP₀₁ mode to the LP₀₂ mode (for example). Curve 21represents the phase-matching relationship of a fiber that yields aBB-LPG at about 1470 nm. A unique feature of the curve 21 is theturn-around point (TAP) in the phase matching relationship. At the TAP avertical line 22 is tangent to the phase-matching curve 21. If a gratingperiod has the value corresponding to the tangent line 22, a broad rangeof wavelengths approximately satisfy the phase-matching relationship.For wavelengths in this range, a BB-LPG results, because the wavelengthsare at or close to resonance near the TAP. In this example, a BB-LPGwith a period of 115 microns (line 22) yields a BB-LPG at about 1470 nm.

[0020] Curve 23 corresponds to the phase-matching relationship of a LPGin a second fiber. From curve 23, one sees that the LPG yields a NB-LPGfor any wavelength greater than about 1300 nm. That is, for wavelengthsgreater than 1300 nm, a vertical line corresponding to a selectedgrating period is never tangent to the phase-matching curve 23. Thus,there is not a broad range of wavelengths that approximately satisfy thephase-matching relationship. For example, line 24, intersects thephase-matching relationship at only one wavelength.

[0021] In this illustration, curve 23 represents a fiber with the sameindex profile as that represented by curve 21, but the fiber associatedwith curve 23 is drawn to a diameter that is 80% of the originaldiameter of the fiber associated with curve 21. Dimensionally scaling afiber in this manner shifts the TAP of the fiber. Thus, a fiber thatyields a BB-LPG when drawn to one diameter, yields a NB-LPG when drawnto a different diameter. Dimensionally scaling is one of several ways toshift or adjust the TAP of the fiber. The same objective may be realizedby inducing a constant index change in the fiber, or by etching theouter diameter of the cladding. While the former technique will resultin shifting the TAP of both a cladding mode as well as a core-guidedmode, the latter will be useful when the HOM employed for the bandpassfilter is a cladding mode.

[0022] To illustrate the characteristics of the fiber sections 11 and 13in the filter of FIG.1, experimentally obtained spectra are shown inFIG. 3. The phase-matching curve 21 of FIG. 2 for fiber 11 (outerdiameter, OD=121 μm) has a TAP at 1540 nm. An LPG written at thecorresponding grating period (112.5 μm) converts the incoming LP₀₁ modeinto the LP₀₂ mode over the entire C-band. The length of this grating is1 cm, with an index perturbation of 5×10⁻³. This illustrates that morethan 99% (>20 dB) of light is converted over a spectral range between1527 nm and 1571 nm.

[0023] More generally, the wavelength range of the BB-LPG may becontrolled by suitably designing a fiber with different dispersionproperties for the fundamental mode and the HOM.

[0024] The bandwidth, Δλ, of the BB-LPG is given by:

Δλ=A×λ _(res) /{square root}{square root over (L×ΔD×c)}

[0025] where ΔD is the difference in dispersion between the two modesthat are being coupled by the BB-LPG, L is the length of the grating,λ_(res) is the resonant wavelength (where maximum coupling occurs), c isthe velocity of light in a vacuum, and A is a constant determined by themaximum coupling strength of the grating. Thus, a variety of broadbandspectra may be obtained utilizing this concept. The wavelength range ina typical BB-LPG may range from 40 to 100 nm.

[0026] Fiber section 13 is a few-mode fiber similar to fiber 11, but isdrawn to an OD of 112 μm. The spectrum of an NB-LPG in this fiber isshown by curve 32 in FIG. 3. This spectrum is for a 5.7 cm long uniformLPG with a grating period of 120 μm, and an index perturbation of1×10⁻⁴. The 3 dB bandwidth is 7 nm and provides 99.6% (24 dB)mode-conversion at the resonant wavelength of 1555 nm. The LPG spectramay be tailored in bandwidth by modifying the dispersive properties ofthe fiber, or the physical parameters of the grating.

[0027] The bandwidth, Ak, of the NB-LPG is given by:

Δλ=A×λ _(res) ² /L×Δn _(g)

[0028] where Δn_(g) is the difference in the group indices between thetwo modes that are being coupled by the NB-LPG, and the rest of theterms are as defined earlier. In addition to control over bandwidth,chirping the period of the grating, or apodizing the index perturbationsof the gratings, can yield a multitude of spectral shapes (e.g.rectangular, Gaussian, etc.).

[0029] With reference to the elements of FIG. 1, the device functions asfollows.

[0030] Essentially all light in an entire communications band passingthrough BB-LPG 12 is converted to the LP₀₂ mode. As the LP₀₂ mode lighttraverses splice 15, that splices fiber section 11 to fiber section 13,the LP₀₂ mode light of fiber 11 is adiabatically converted into the LP₀₂mode of fiber 13. Then, the NB-LPG 14 selects a desired narrow portionof the spectrum and converts that back into the LP₀₁ mode. This resultsin a bandpass filter, with properties shown by curve 35 in FIG. 4.

[0031] The device illustrated in FIG. 1 provides a general platform forbuilding band-selection as opposed to band-rejection filters with LPGs.The first mode-converter, BB-LPG 12, only serves as a device thatprovides a spatially modified input for NB-LPG 14, and does not definethe spectral characteristics of the bandpass filter. The spectrum of thefilter is defined by the inverted spectrum of the NB-LPG 14. Further,instead of only one NB-LPG 14, several NB-LPG may be added in series.Alternatively, the NB-LPG may be designed to possess multiple narrowbandresonances. This enables the prospect of spectral shaping in theband-pass configuration by varying the spectral properties of one ormore NB-LPG 14.

[0032] The device just described has an advantage over conventionalfilters using LPGs in that no core-blocking element is used. This avoidsthe loss inherent in devices with that configuration. For the purpose ofdefining this distinction, the transmission path between the BB-LPG andthe NB-LPG functions adiabatically. Furthermore, the transmission pathcoupling the LPGs does not include an active attenuation element, i.e.an element that has an intended and deliberate function of attenuatinglight. It is not intended that a conventional splice, which is designedfor minimum light attenuation as is not, in this context, an activeattenuation element.

[0033] The splice element 15 in FIG. 1 adiabatically transforms the HOMof fiber 11 (after traversing the NB-LPG) into a HOM of fiber 13. Thismay be achieved by inducing a heat profile along the splice and/ortapering one fiber with respect to the other to ensure that the two HOMscouple efficiently. In the embodiment shown in FIG. 1, the two fibers 11and 13 are shown with a physical splice joining them, i.e. fibers 11 and13 are separate fibers. The physical splice 15, and the attendant lossin that splice, may be avoided by making a single fiber with twodistinct transmission characteristics. For example, the diameter of thefiber may be made to vary longitudinally along the fiber length, i.e. afirst section with a first diameter, and a second section with a seconddiameter. Alternatively, the cladding may be selectively modified fromone portion of a single fiber to another. Therefore, the functionaloperation of splice 15 is the important aspect, and may be defined as ameans for effecting a change from a HOM with one characteristic to acorresponding HOM with a different characteristic.

[0034] Since uniform LPGs are not dispersive filters, the bandpassfilters described here are not dispersive.

[0035] The filter of the invention may also be tuned by inducing a shiftin the phase-matching curve for the NB-LPG (curve 23 of FIG. 2).Alternatively, doping the cladding of the fiber, or coating the outsidecladding of the fiber with an electro-optic or non-linear optic materialallows electrical or optical control of the resonant wavelength of thebandpass filter.

[0036] The bandpass filter of the invention may be used in a variety ofsystems. For example, a variety of laser devices can be tuned using thebandpass filter of the invention as an intracavity element. In aconventional laser cavity, defined by two narrowband reflectors, onehigh reflector and one weak reflector (output mirror), the two narrowband reflectors are replaced by two broadband reflectors (one high andone weak) with the bandpass filter of the invention in the cavity. Thelasing wavelength may be adjusted by tuning the filter. Multiple lasingwavelengths can be produced by having one or more NB-LPGS in thebandpass filter that have multiple resonances.

[0037] An illustration of this generalized form of laser is shown inFIG. 5, where the gain fiber is shown at 38, high reflector at 39, weakreflector at 41, input signal at 43, laser pump at 44, and WDM forcombining signal and pump at 45. The bandpass filter of the invention,shown generally ar 46, and comprising BB-LPG 47, NB-LPG 48, and splice49, is placed inside the laser cavity as shown.

[0038] A specific form of laser, a cascaded Raman fiber laser, isespecially adapted for use with the filter of the invention. Thesedevices can be made to operate at multiple wavelengths by employingmultiple sets of Bragg gratings that comprise narrowband high-reflectorsand output couplers/weak reflectors. These pairs of Bragg gratingsdefine laser cavities, and the resonant wavelength of the narrowbandgrating defines the lasing wavelength.

[0039] An embodiment of this device is illustrated in FIG. 6. The Ramanresonator 51 comprises fiber 52 and gratings 53, 54. In the conventionaldevice, the resonator is bounded on both sides by a narrow band grating.In this embodiment, the resonator is bounded by broadband high-reflector48, and weak reflector 49 gratings. Now, the resonant wavelength iscontrolled by a bandpass filter 56 of the invention. The bandpass filter56 comprises, as described earlier, BB-LPG 57, NB-LPG 58, and splice 59.Tuning is achieved by tuning the NB-LPG of the bandpass filter. SinceLPGs (and therefore the LPG bandpass filters) are widely tunable bymethods described earlier, a swept wavelength source, useful in Ramanamplified optical communications systems, would be easily realizable.For more details on these systems, and for swept wavelengthimplementations in these systems, reference is made to co-pendingapplication Ser. No. 10/098,200 filed Mar. 15, 2002, entitled “WidebandRaman Amplifiers”, which is incorporated herein by reference. Moregenerally, multiple NB-LPG 58 may be added in series to providesimultaneous tunable operation of the device over several lasingwavelengths.

[0040] The bandpass filters of the invention are also well adapted foruse with self-phase modulation (SPM) based optical regenerators. Thesedevices are relatively simple and typically comprise a non-linear fiberassociated with a bandpass filter (see U.S. Pat. No. 6,141,129). Thesignal is launched into the non-linear fiber section, thereby inducingSPM in the signal. The effect is to broaden the spectrum of the portionsof the signal that exceed a selected intensity value. The bandpassfilter then selects a predetermined portion of the broadened spectrum.Ideally the bandpass filter for this application should be tunable toaccount for temporal changes in the spectrum. It is also desirable thatthe filter be dispersion-less. These desiderata are satisfied by thebandpass filter of the invention.

[0041] A schematic representation of an optical regenerator using thebandpass filter of the invention is shown in FIG. 7. The non-linearfiber section is shown at 62, and the bandpass filter, comprising BB-LPG67, NB-LPG 68, and splice 69, is shown generally at 66.

[0042] Pre-amplifiers for optical receivers employ noise filters forremoving spontaneous emission noise from the received signal. This noiseoriginates from amplifiers along the transmission path. The noise istypically over the entire spectral range including the signalwavelength, and substantial sidebands of the signal. This backgroundnoise degrades the signal-to-noise ratio of the signal to the receiver.A bandpass filter could selectively attenuate noise outside the signalwavelength, and therefore improve the optical signal-to-noise ratio, butwould be especially effective if the bandpass filter weredispersion-less. That characteristics is provided by the bandpass filterof the invention.

[0043] An optical pre-amplifier system as just described is shown inFIG. 8, where the optical amplifier 72 is followed by a bandpass filter,shown generally at 76, comprising BB-LPG 77, NB-LPG 78, and splice 79.The filtered signal is then introduced into a receiver, represented inthe figure by light sensitive diode 81.

[0044] The invention is described above using optical fibers forimplementing the bandpass filter of the invention. Similar devices maybe constructed using other forms of waveguides, for example planarwaveguides in optical integrated circuit (OIC) devices.

[0045] The long period gratings described here may be formed by varioustechniques. A common approach is to write the gratings into a Ge dopedfiber using UV light. However, other methods may also be used. Forexample, microbend induced LPGs are suitable. These can be realized withacousto-optic gratings, arc-splicer induced periodic microbends, or bypressing the fiber between corrugated blocks that have the requiredgrating periodicity.

[0046] Various additional modifications of this invention will occur tothose skilled in the art. All deviations from the specific teachings ofthis specification that basically rely on the principles and theirequivalents through which the art has been advanced are properlyconsidered within the scope of the invention as described and claimed.

We claim:
 1. An optical filter comprising: a. an optical waveguidehaving a first portion and a second portion, b. a BB-LPG in the firstportion of optical waveguide, c. a NB-LPG in the second portion ofoptical waveguide, and d. a coupler for serially coupling the firstportion and the second portion.
 2. The optical filter of claim 1 whereinthe first portion of optical waveguide and the second portion of opticalfiber each comprise optical fiber.
 3. The optical filter of claim 2wherein the coupler comprises a fiber splice.
 4. The optical filter ofclaim 2 wherein the first portion of optical fiber has a diameterdifferent from the diameter of the second portion of optical fiber. 5.An optical filter comprising: a. an optical waveguide having a firstportion and a second portion, the first and second portions beingserially connected, b. a first LPG for converting signal lightpropagating in mode LP₀₁ into a signal propagating in mode LP_(m,n), c.an optical coupler configured to transmit a signal light propagating inmode LP_(m,n) in the first portion to LP_(r,s) mode light in the secondportion, wherein m=r and n=s, d. a second LPG for converting a selectedband Δλ₂ of the signal light propagating in mode LP_(r,s) in the secondportion to light propagating in mode LP₀₁.
 6. The optical filter ofclaim 5, further comprising: a lightwave signaling means for introducinga lightwave signal into the first portion, the introduced lightwavesignal having a bandwidth Δλ₁ and propagating as mode LP₀₁.
 7. Theoptical filter of claim 5 wherein the first portion of optical waveguideand the second portion of optical waveguide each comprise optical fiber.8. The optical filter of claim 7 wherein the first portion of opticalwaveguide and the second portion of optical waveguide each compriseoptical fiber.
 9. The optical filter of claim 6 wherein the opticalcoupler comprises a fiber splice.
 10. The optical filter of claim 2associated in an optical system comprising a cascaded Raman resonatorhaving a Raman resonator cavity, wherein the optical filter is seraillycoupled to the Raman resonator.
 11. The optical filter of claim 2associated in an optical system comprising an optical amplifier, whereinthe optical filter is serially coupled to the optical amplifier.
 12. Alaser comprising: a. a resonant cavity, b. tuning means in the resonantcavity, the tuning means comprising: i. an optical waveguide having afirst portion and a second portion, the first and second portions beingserially connected ii. a BB-LPG in the first portion of opticalwaveguide, iii. a NB-LPG in the second portion of optical waveguide, andiv. a coupler for coupling the first portion and the second portion.